In the field of aerospace, linear flight control actuators currently in use employ hydraulic cylinders or ballscrews to provide positional control and to react aerodynamic loads. Actuators constructed of hydraulic cylinders are used extensively but are a maintenance issue because of their high potential for fluid leakage. Leakage can occur in distribution line connections, valve blocks, transfer ports, piston seals, etc.
Ballscrews have been used in many secondary flight control systems, such as flaps and slats. Ballscrews have also been used in primary flight control surfaces, such as rudders and ailerons, but to a lesser degree because of the perception in the aerospace industry that ballscrew actuators are prone to jam failures. Although the reliability statistics indicate that jam failures for ballscrew actuators are extremely rare, the perception remains.
Ballscrew actuators offer many advantages relative to other approaches, such as hydraulic cylinders, in terms of higher stiffness, lower weight, lower cost, and packaging flexibility.
Typical of the ballscrew actuators noted above is that shown and described in the Grimm U.S. Pat. No. 4,603,594 ('594). The '594 patent is directed to a fail-safe actuator that includes a single ballscrew in combination with planetary gearing, disk brakes, a solenoid, and motor to actuate control surfaces on an aircraft.
A non-jamming screw actuator system is shown and described in the Klopfenstein U.S. Pat. No. 4,745,815 ('815). The '815 patent achieves a degree of jam tolerance by releasing and engaging multiple electromagnetic clutches to selectively change the torque reaction points in a ballscrew/gear screw actuator assembly. This arrangement requires a sophisticated electronic controller to sense appropriate reactions to failure modes.
Another ballscrew actuator typical of the prior art is that shown and described in the Teske et al U.S. Pat. No. 4,637,272 ('272). The '272 invention employs a single ballscrew and obtains failure mode protection by the provision of redundancy through the use of independently driven ballnuts on the same screw. The ballnuts are driven by separate motors.
The invention to be described hereinafter distinguishes over the prior art above in a most advantageous manner in that the ballscrew actuator of the invention provides a simple mechanical approach utilizing dual ballscrews and a differential mechanism to provide jam tolerance and automatic rate compensation in the unlikely event of a jam in the actuator.